Normalisation
Normalisation organises tables to remove redundancy and avoid anomalies:
- Insertion anomaly: cannot add data without other, unrelated data.
- Update anomaly: the same fact must be changed in many rows.
- Deletion anomaly: deleting a row loses other useful facts.
A functional dependency X → Y means the value of X decides the value of Y.
| Normal form | Rule |
|---|---|
| 1NF | Every cell holds a single (atomic) value. No repeating groups. |
| 2NF | In 1NF and no partial dependency: no non-key column depends on only part of a composite key. |
| 3NF | In 2NF and no transitive dependency: a non-key column must not depend on another non-key column. |
| BCNF | Stronger 3NF: every determinant is a candidate key. |
Example. A table Student(RollNo, Name, CourseID, CourseName) repeats the course name for every student. CourseName depends on CourseID, not on the student. Split it into Student(RollNo, Name, CourseID) and Course(CourseID, CourseName).
Denormalisation adds redundancy on purpose to make reading faster.
Keep practising
Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.
Prepared for exam revision from standard computer textbooks, Microsoft documentation and official sources. Features differ between Excel and Word versions (2016, 2019, 2021, 365) so check which version your exam follows. Found a mistake? Tell us.